After a borehole has been drilled into the earth, a string of steel casing or liner is lowered and set therein. One drillable shoe and possibly one drillable collar having an upwardly closing check valve are mounted on or near the lower end of the string to prevent back flow. After the liner has been suspended by a hanger apparatus near the lower end of a previously run casing string, cement slurry is pumped down the interior thereof and out into the borehole via the check valves where it flows up in the annulus outside the liner up to a desired level. The drilling mud that was standing in the well prior to cementing is displaced and circulated out of the well during the casing setting and cementing steps. When the cement has hardened, it seals off the annular space between the outside of the liner and the surrounding well bore wall and prevents migration of formation fluids therealong.
It is highly desirable to protect the cement slurry from contamination by the drilling mud as the slurry is being pumped into the well. The usual practice to protect it is to place a first plug ahead of the cement column which provides a separation between the lower end of such column and the mud, and to place a second plug which performs the same function at the top of the column. Each plug typically has a series of upwardly facing elastomer cups whose outer edges engage the inner walls of the liner to provide sliding seals and wipers. When the first plug lands against a float shoe at the bottom of the liner, a passage is opened up through it which enables cement to be pumped into the annulus. Eventually the second plug lands against the first plug as the displacement is completed. The check valves in the float shoes prevent back flow of the cement into the casing or liner during the time that it takes for the cement to set up. During downward movement the outer edges of the cups of the second plug wipe or scrape the cement off of the inner walls of the liner so that no deposits are left. Once the cement has hardened, the plugs and cement shoes can be drilled out.
Heretofore, wiper plugs used in cementing liners have been designed such that cement slurry and other fluids could be pumped through a flow passage in the plug itself, which requires complicated valve systems to open and close this passage. This complexity has created difficulties in and of itself, and has resulted in plug structures that are difficult to drill out at the end of the cementing operation. The inclusion of such valve structures also has reduced the performance characteristics of such plugs, particularly when the liner hanger and wiper plug launching system are utilized on directional or horizontal sections of a well. For these reasons a person skilled-in-the-art would not run multiple plugs while cementing liners. Only the top shut-off plug would be used in most instances in order to reduce the risk of any mechanical failure. However, not having bottom wiper plugs increases substantially the risk for the cement slurry to channel through the mud inside the liner and therefore be completely contaminated before reaching the annulus. This phenomenon has been responsible for a large number of liner cementing failures.
The general object of the present invention is to provide a new and improved plug launching system of the type described that obviates the above-mentioned problems with prior systems.